1. Field of the Invention
The present invention relates to a light emitting device comprising a plurality of pixels each having a light emitting element and a means for supplying current to the light emitting element.
2. Description of the Related Art
Since a light emitting element emits light by itself, it is highly visible and does not require a back light which is needed in a liquid crystal display device (LCD). Therefore, it is suitably applied to thin devices and not restricted in viewing angle. Because of these advantages, a light emitting device having a light emitting element has recently been drawing attentions as an alternative display device to a CRT and an LCD. It is to be noted that a light emitting element in this specification indicates an element whose luminance is controlled by current or voltage, and it includes an OLED (Organic Light Emitting Diode) or an MIM electron source element (electron discharge element) and the like which is used in an FED (Field Emission Display).
Also, a light emitting device of the invention includes a panel and a module obtained by mounting an IC or the like onto the panel. More generally, the invention relates to an element substrate which corresponds to the one before the completion of a panel in manufacturing steps of the light emitting device, and the element substrate comprises a plurality of pixels each having a means for supplying current to a light emitting element.
OLED which is one of the light emitting elements includes an anode layer, a cathode layer, and a layer containing an electric field light emitting material (hereinafter referred to as an electroluminescent layer) that generates luminescence (electroluminescence) when an electric field is applied thereto. The electroluminescent layer is provided between an anode and cathode, and it comprises a single or multiple layers. These layers may contain an inorganic compound. The electroluminescence in the electroluminescent layer includes a light emission (fluorescence) when a singlet exciting state returns to a ground state and a light emission (phosphorescence) when a triplet exciting state returns to a ground state.
Next, the configuration of a pixel of a general light emitting device and its drive will be described in brief. A pixel shown in FIG. 9 comprises a switching transistor 900, a driving transistor 901, a capacitor 902, and a light emitting element 903. The gate of the switching transistor 900 is connected to a scan line 905. Either the source or drain of the switching transistor 900 is connected to a signal line 904, and the other is connected to the gate of the driving transistor 901. The source of the driving transistor 901 is connected to a power supply line 906, and the drain of the driving transistor 901 is connected to the anode of the light emitting element 903. The cathode of the light emitting element 903 is connected to a counter electrode 907. The capacitor 902 is provided for storing a potential difference between the gate and source of the driving transistor 901. Also, the predetermined voltages are applied to the power supply line 906 and the counter electrode 907 from a power supply and each has a potential difference.
When the switching transistor 900 is turned ON by a signal from the scan line 905, a video signal that is inputted to the signal line 904 is inputted to the gate of the driving transistor 901. The potential difference between a potential of the inputted video signal and that of the power supply line 906 corresponds to a gate-source voltage Vgs of the driving transistor 901. Thus, current is supplied to the light emitting element 903, and the light emitting element 903 emits light by using the supplied current.